Satellite television, or TVRO, signal downlink equipment is presently characterized by the use of horn antennas of the type which have become known as scalar feed horns. Scalar feed horns generally consist of a cylindrical waveguide, the radiating aperture of which is surrounded by a plurality of concentric rings. Such feed horns are positioned at the focal point of a suitable reflector dish for microwave signals transmitted from a satellite in geosynchronous orbit about the earth. Until recently, TVRO satellite signals have been transmitted principally in the operating frequency band of from 3.7 to 4.2 GHz, an operating band referred to by persons in the field as the C-band. Horn antennas used for reception of TVRO signals have heretofore had to have acceptable performance characteristics over the C-band but not necessarily at other microwave frequencies.
Because of the dual polarized nature of TVRO satellite signals, moreover, horn antennas utilized for TVRO have also had to be able to switch, upon demand, from one polarization of the incoming signal to the other. This requirement has given rise to the common use of a small rotatable metal probe assembly located at the bottom or back of the waveguide and coupled electrically to a standard WR229 rectangular waveguide. Such a feed horn is shown and described in U.S. Pat. No. 4,414,516 to Taylor Howard.
In the past few years, some TVRO satellite channels have, for many reasons, been transmitted at frequencies within the range of from 11.7 to 12.2 GHz, a frequency band referred to by persons in the field as the Ku-band. Thus, some satellite television stations are transmitted in the C-band range, while others are transmitted in the Ku-band. Accordingly, it has become desirable today for TVRO earth stations to be comprised of equipment capable of receiving and processing both C-band and Ku-band signals simultaneously.
Microwave waveguide junctions consisting of coaxial waveguides for simultaneous reception of independent frequency ranges have been known heretofore. For example, U.S. Pat. No. 3,508,277 to Ware et al discloses the use of two cylindrical waveguides mounted coaxially. Ware et al however do not utilize rotatable coupling probes to achieve efficient and low cost polarization switching and do not address the problems associated with the use of such coupling techniques. U.S. Pat. No. 4,041,499 to Liu et al discloses a waveguide antenna in which inner and outer waveguides are side-fed by fixed coaxial probes. The inner waveguide is fed with a monopulse signal in the sum or in-phase mode and the outer waveguide is similarly side-fed with a monopulse signal in the difference or out-of-phase mode. Such a structure is not intended for use with TVRO signals and does not address the problems of cost effective dual frequency TVRO reception. U.S. Pat. No. 4,740,795 to Seavey discloses a dual frequency antenna feed assembly having a pair of coaxial waveguides. In Seavey, however, the rectangular launch box for the high-band signal is mounted directly at the bottom of the high-band waveguide and exits radially from about the center of the surrounding low-band waveguide. Such an arrangement necessitates the use of four coaxial transmission lines spaced around the periphery of the low-band waveguide to transform the mode of and to conduct the low-band signal past the high-band launch box to a polarization rotator at the back of the low-band waveguide. Seavey therefore requires additional transformations of the low-band signals, is expensive to produce, time consuming to assemble and generally has too large a noise temperature for highly effective TVRO reception. In another commercially available dual frequency feed for TVRO, the Ku-band signal launch box is mounted on the scaler rings making the illumination characteristic of the feed unadjustable. In addition, the high-band signal is carried radially outwardly through the low-band waveguide line by a coaxial transmission line which traverses the throat of the low-band waveguide in a direction parallel to the electric field within the waveguide. Accordingly, the mechanism is complex, expensive and the way in which the high-band signal is extracted tends to disturb the signal and to increase the noise temperature of the device as a whole.